As areal densities increase, smaller bit cells are required in the magnetic medium (track width and bit length). However, superparamagnetic instabilities become an issue as the grain volume (i.e., the number of grains in the media per bit cell) of the recording medium is reduced in order to control media noise for high areal density recording. The superparamagnetic effect is most evident when the grain volume V is sufficiently small that the inequality KuV/kBT>70 can no longer be maintained. Ku is the material's magnetic crystalline anisotropy energy density, kB is Boltzmann's constant, and T is absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the stored bits. Therefore, as the grain size is decreased in order to increase the areal density, a threshold is reached for a given material Ku and temperature T such that stable data storage is no longer feasible.
Bit-patterned magnetic media (BPM) have been proposed as a means for preventing magnetization reversal due to the superparamagnetic effect. A patterned medium may include a plurality of discrete, independent regions of magnetic material that form discrete, independent magnetic elements, which function as recording bits in the medium. The magnetic bits or elements are arranged in a regular pattern over the medium substrate. Each bit or element has a small size and magnetic anisotropy such that, in the absence of an applied magnetic field, the magnetic moment of each magnetic bit or element is aligned along the same magnetic easy axis.
In order to write at very high areal densities, the size of the write pole may be decreased to increase the track density. However, the strength of the write field achievable by the writer decreases with a reduction in the write pole width. The write field is reduced even further by conventional side shielding assemblies. In addition, as the write pole moves radially across the medium, the field profile from the write pole tip becomes skewed relative to the magnetic bits or elements in the magnetic medium. This can lead to writing errors and inadvertent erasure of previously recorded data. While effects on the writing process due to skewing can be somewhat mitigated by increasing the length of the write pole, the head-to-medium spacing (HMS) and medium thickness cannot be scaled down to the same extent as the write pole width to further remedy skewing effects.